Neutron diffraction and quasielastic neutron scattering studies of films of N-alkanes and a branched alkane adsorbed on graphite

Abstract

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT REQUEST OF AUTHOR.] Neutron diffraction and quasielastic neutron scattering have been used to study the structure and dynamics of films of n-alkanes and of a branched alkane adsorbed on graphite. The structures of the intermediate-length alkane n-tetracosane and of the branched alkane squalane adsorbed on graphite were studied below room temperature using neutron diffraction. Comparison of the n-tetracosane and the squalane monolayers reveals that both undergo a phase transition from a more-ordered crystalline phase to a less-ordered "smectic" phase at low temperature, [approximately]210 K for n-tetracosane (n-C[subscript 24]D[subscript 50]) and [approximately]215 K for squalane (C[subscript 30]H[subscript 62]). However, the squalane monolayer is less ordered translationally than the n-tetracosane monolayer at every temperature studied as indicated by the smaller coherence length Lb of the squalane film. This is presumably due to the side branches along the squalane molecules that prevented them from translationally ordering translationally as well as the linear tetracosane molecules. Quasielastic neutron scattering studies of monolayer, bilayer, and trilayer nheptane (n-C[subscript 7]D[subscript 16]) above a crystalline n-dotriacontane monolayer adsorbed on graphite investigated the dynamics within these heptane layers. These studies found that the heptane layers exhibited both a slow motion dominating at low wave vector transfer Q and a faster motion dominating at higher wave vector transfer Q. The slow motion within the heptane layers at all coverages is similar to that measured for the n-dotriacontane monolayer alone. The faster motion measured for all coverages of heptane was comparable to the motions measured for bulk heptane. Two coverages of n-tetracosane monolayers, 1.00 and 1.16 layers based on the lattice parameters of n-tetracosane in its "smectic" phase, were studied by high-energy resolution quasielastic neutron scattering. Uniaxial rotational motion of the nearly alltrans n-tetracosane molecule begins in the monolayer crystalline phase at [approximate sign]160 K, independent of coverage. Uniaxial rotational motion continues up to a temperature of [approximately]230 K for the low-coverage sample and up to a temperature of [approximately]260 K for the highcoverage sample, as indicated by the Q-independence of the quasielastic scattering. At [approximately]260 K and above, the width of the quasielastic scattering of the low-coverage n-tetracosane sample begins to increase with increasing Q, indicating a more complex combination of translational and rotational diffusive motion in the "smectic" phase of the sample. The high-coverage sample continues to exhibit uniaxial rotational motion. The decrease in the quasielastic intensity and molecular dynamics (MD) simulations indicate that many of these motions become too fast to lie within the instrumental energy window above 285 K. However, the observed quasielastic intensity increases again at 330 K. MD simulations indicate that intramolecular and rotational diffusion of more globularly shaped C24 molecules, which contain many gauche defects, is being observed.